It is an object of silver halide-based color photographic materials to reproduce colors in both an accurate (in terms of hue) and vivid (in terms of saturation) manner. In practice, the reproduction of color by such materials is limited in two ways. First, the sensitivity of the silver halide emulsions to a desired single light color is not perfect and they will absorb some amount of light of undesired color. This leads to formation of dye in the wrong color record resulting in less pure hues. For example, the red sensitivity of the emulsions generally occurs at longer wavelengths than the human eye. If the red sensitivity of the film is moved closer to the eye maximum sensitivity, its sensitivity to green light also increases. Thus in such situations, the red sensitive layer is partially exposed during green light exposures leading to the formation of some cyan dye along with magenta dye. This alters the hue of the image and decreases its saturation. Second, the image dyes formed are not perfect in hue and have unwanted side absorbencies. Thus, some density in the unwanted color regions is formed in addition to the desired density, again degrading color saturation. Finally in some circumstances, it is desirable to increase color saturation to a greater degree than the actual image in order to make the image visually more pleasing.
It is well known that color reproduction of such materials can be partially controlled by the use of imagewise development inhibitor releasing (DIR) couplers. During development, DIR couplers react with oxidized developer to release an inhibitor fragment or a precursor of an inhibitor fragment which can diffuse out of that layer and into a different color record where inhibition occurs. This has the overall effect of reducing the amount of dye formed in one color record as a function of exposure of another and can effectively be used to manipulate hue and increase color saturation. This process is called interimage. For example, a film with a DIR coupler in the green layer and given a mostly green exposure will cause a decrease in development in the red record due to the action of the inhibitor released in the green. This causes less cyan dye to be formed than when the inhibitor was not present. The final green image will have less red density and its overall saturation will be increased. It should be noted that all possible colors are not weighted equally in terms of creating a pleasing overall image and that the reproduction of some key colors (for example, flesh tones, green grass, blue sky, etc.) are more important than others.
The creation of interimage effects with DIR couplers is deficient in a number of ways. First, the inhibitor fragment (or precursor) released from the DIR coupler is free to diffuse in all directions. Thus, the inhibitor can affect both of the other color records, even if it was desired to only affect one. For example, putting the DIR coupler in the green will decrease the amount of blue development as well as the red. The amount of interimage effects on the blue and red records from the green are linked and cannot be manipulated separately. This non-specificity of interimage effects limits the ability to control and manipulate color reproduction of the key colors.
Second, the fragment released from the DIR will cause inhibition in the layer in which it is released. This can lead to over-inhibition of the layer in which the DIR coupler is located resulting in low contrast and a loss in sensitivity to light, particularly with strong inhibitor fragments. It is possible to avoid this in part by using milder inhibitors or by using timing groups to delay the introduction of the free inhibitor fragment. In such situations, the diffusion pathlength of the inhibitor fragment is increased and seasoning of the fragments into the developer becomes a problem. In order to avoid these seasoning effects, mild inhibitor fragments often have a hydrolyzable substituent which, upon hydrolysis in the developer solution, renders them inactive after a period of time. Examples are shown in U.S. Pat. Nos. 4,782,012, 4,477,563, 4,937,179, 5,004,677, DE-A 3909486, DE-A-3209486, EP-A-167,168, EP-A-488,310, EP-A-440,466 and EP-A-219,173.
Substituted purines and 1H-pyrazolo[3,4-d]pyrimidines are known to be useful in photographic systems as antifoggants; for example, as in Japanese Patent Applications JP 07-281345A2; JP 03-013934A2; JP 03-138639A2; JP 04-107446A2; JP 04-067140A2 and JP 05-127290A2 and as described in E. J. Birr, Stabilization of Photographic Silver Halide Emulsions, Focal Press, Ltd, 1974, pp 82-83. As antifoggants, these materials are generally at least partially water soluble or soluble in water-miscible solvents such as methanol and are added directly to silver emulsions before coating of the film or added directly to the developer solutions. Purines are also described in Def. Publ. U.S. Pat. No. 877,011 to be useful as silver pi-complex stabilizers in photographic systems. Purines used as inhibitor fragments as part of a DIR are disclosed in U.S. Pat. No. 3,933,500. Purines as a blocking group for the release of photographically useful groups during processing are described in JP 04-186344A2 and EP-335319A2. Derivatives of 6-aminopurines as addenda in high contrast black and white media containing not DIR couplers have been described in JP10-104784A2; JP10-228077A2; JP09-106024A2 and JP04-336538A2. U.S. Pat. Nos. 5,411,929 and 5,328,799 describe the use of purines, including 6-dodecylaminopurine (ClogP=6.21), in coupler-free thermally processed image recording materials.
U.S. Pat. No. 5,508,154 describes the use of 5/5 bicyclic heterocycles in which one ring is a 1,2,3-triazole, and both rings together contain a minimum of 4 nitrogen atoms as antifoggants in systems that contain inhibitor releasing couplers. Of the examples shown, these heterocycles have an average ClogP of 1.53 with a maximum of 5.67 (example A-7). The patentee also notes that 6/5 membered bicyclic heterocycles with 4 nitrogen atoms do not produce the desired result.
DE 1 95 07913 A1 describes the use of ballasted benzimidazoles to improve granularity particularly with certain pyrazolone image couplers.
A problem to be solved is to provide a color photographic element having improved color reproduction.